The Physics behind the Modulation of Thermionic Current in Photodetectors Based on Graphene Embedded between Amorphous and Crystalline Silicon

Abstract

In this work, we investigate a vertically illuminated near-infrared photodetector based on a graphene layer physically embedded between a crystalline and a hydrogenated silicon layer. Under near-infrared illumination, our devices show an unforeseen increase in the thermionic current. This effect has been ascribed to the lowering of the graphene/crystalline silicon Schottky barrier as the result of an upward shift in the graphene Fermi level induced by the charge carriers released from traps localized at the graphene/amorphous silicon interface under illumination. A complex model reproducing the experimental observations has been presented and discussed. Responsivity of our devices exhibits a maximum value of 27 mA/W at 1543 nm under an optical power of 8.7 μW, which could be further improved at lower optical power. Our findings offer new insights, highlighting at the same time a new detection mechanism which could be exploited for developing near-infrared silicon photodetectors suitable for power monitoring applications.

Keywords: encapsulation; graphene; near infrared; photodetector; silicon photonics.

Figure 1

(a) Device sketch and (b) detection mechanism described by the energy band diagram for single-layer graphene under NIR illumination the traps at the a-Si:H/Gr interface release charge carriers into Gr, changing the Schottky barrier of the Gr/c-Si junction and hence the current flowing through the device.

Figure 2

(a) Optical field distribution in the proposed photodetector as a function of position for a 208 nm-thick a-Si:H layer (z = 0 corresponds to the air/a-Si:H interface). (b) Theoretical Gr optical absorption for different thicknesses of the a-Si:H layer (inset: Gr absorption as a function of the wavelength for various a-Si:H thicknesses).

Figure 3

(a) Top view of the device taken by optical microscope. (b) Raman spectra of bare patterned graphene on silicon oxide (blue spectrum) and Gr capped with a-Si:H (red spectrum).

Figure 4

(a) I-V curve of the device and (b) time dependence of the measured dark current flowing through the device at a fixed voltage of −21 V.

Figure 5

Dark current (i_D) and current generated by NIR illumination (i_L) measured for an optical power of (a) 26.5 µW and (b) 8.7 µW at 1543 nm. (c) Measured responsivity at 1543 nm vs. negative voltage applied for various optical power incidents on the device in Figure 1a. (d) Measured responsivity around 1550 nm vs. negative voltage applied for various optical powers incident on the Gr/c-Si Schottky junction before the a-Si:H deposition.

Figure 6

Responsivity at 1543 nm vs. optical power at −21 V (inset: efficiency-lifetime carrier product as function of the optical power).

Figure 7

Electrical circuit schematizing the electrical behavior of the device provided with a current generator representing the photogenerated current (in red) under NIR illumination.